This invention relates to well-logging methods apparatus and, more particularly, to acoustic well-logging methods and apparatus facilitating accurate and automatic detection of the onset of an electric signal generated in response to reception of an acoustic signal indicative of the nature of the formations traversed by the well bore.
Acoustic well-logging methods and apparatus are usually designed to measure the transmit time, per unit of length, of acoustic compression waves in earth formations traversed by boreholes, by means of an acoustic measurement wave moving between a transmission point and a reception point.
French Pat. No. 1,349,989 discloses such a method employing two receivers arranged a certain distance from each other, between two transmitters. With such a device, one measures the transit time between the two receivers for measurement waves transmitted respectively by the first and by the second transmitter. By taking the average (i.e., the half-sum) of the transit times thus obtained, an accurate measurement is obtained independent, in particular, of the errors introduced by the tilting of the probe in the borehole.
There are various methods for measuring the transit time of an acoustic wave between a transmitter and a receiver. A current one, disclosed in the patent mentioned above, involves triggering a timing-pulse counter the instant a given half-wave of a synchronous pulse representative of the operating point of the transmitter is applied to the counter's starting circuit and of stopping the counter when the acoustic measurement waves are received and a corresponding half-wave in the signal generated by the receiver is applied to the counter's stopping circuit. The standard shaping of the synchronous transmission pulse is easy and, consequently, the starting of the counter takes place under favorable conditions. This is not true of the reception signal. Because of the presence of noise superimposed on the measurement signals generated by the receiver, a detection threshold is set for the reception signal so that the counter is not likely to be stopped accidentally by the noise signals. Such noise signals are of two kinds: those picked up by the receiver and those picked up in the cable (crosstalk). The noise signals generated by the receiver are intermittent and can reach a very high amplitude. Such is the case when the probe strikes against the wall of the borehole and, to a lesser extent (longer duration and lower amplitude), when the probe rubs against a borehole wall which has an uneven surface or cavities. The crosstalk noise picked up in the cable is constantly present, and its amplitude is relatively low compared with that of the noise produced by probe impact against the wall of the borehole.
In practice, when one uses a well-logging apparatus without any noise-compensation device, the operator verifies from time to time, on an oscilloscope, the quality of the signals received at the surface, and, when he notices that, within the region being surveyed, the average noise superimposed on the measurement signals increases or decreases significantly, he modifies accordingly the threshold at which the useful half-wave of the reception signal is detected. Under these conditions, the instant the reception signal exceeds the threshold level thus set, a stop signal is applied to the counter. Consequently, it is essential that the operator monitor the conditions under which the measurement is carried out. Such monitoring is tedious, and, in spite of the vigilance of the operator, it often happens that the threshold set for the detection of the measurement signals corresponds only belatedly to the instantaneous noise conditions at probe level, thereby leaving room for transit time measurement errors.
To remedy this situation, U.S. Pat. No. 2,857,011 discloses measuring the amplitude of the noise between the instant the measurement acoustic wave is transmitted and the instant it is received. When the noise amplitude thus measured exceeds a certain threshold, empirically set once and for all in accordance with certain characteristics of the probe and of the formations, the conducted measurement is considered to be poor and is not transmitted to the following circuits. Such a device has many drawbacks. First, since the received measurement signal varies greatly according to the nature of the different earth formations over which the probe is moved, the setting of a single noise threshold, valid for all measurement conditions, cannot be optimized. Moreover, if the measured noise exceeds the set threshold, the conducted measurement is canceled, even if it was good, which can occur, in particular, when the amplitude of the useful half-wave of the measurement signal is significantly higher than the noise threshold.
A further observation clearly shows that the technique described in the cited patent is not really suited to the problems to be solved. The transit time of an acoustic wave, in the frequently-encountered case where the transmitter and the receiver are about one meter apart, varies from about 125 to 500 microseconds. The noise measurement gate disclosed in the cited patent, which is to operate between the transmission and reception instants, must have an open time substantially shorter than the minimum value of this transit time, i.e., no longer than about 100 microseconds. However, the noise generated by the rubbing of the probe on the walls of the borehole can have a relatively low frequency spectrum (of the order of a few kilohertz). The measurement of the noise amplitude following a single sampling of such brief duration in relation to the average period of this same noise cannot provide reliable information as to frequency components which cannot be measured during such a short interval.